Hi guys! When the OL moves a "revealed" monster (not spawned but already existing on the map in the line of sight of the heroes), that is hiding another one, and make the hidden monster seen by the heroes, can the OL immediately activate and use it or does he have to wait his next turn?

Help me!!!

Just to make sure I have it clear: You are talking about monsters that the OL placed on the map as a result of the Heroes opening the door to an unrevealed area?

The OL can activate and use any of the monsters that are on the map either as a result of spawning or a revealed area during his turn (baring effects on the monster that prevents the monster from taking a turn, like Stun for regular monsters). Heroes having LOS to a monster has no impact on the activation.

thanks...and when the heroes discover a new area, does th OL have to show ONLY the monsters in the LOS of the heroes or every monster in the new area, even those are not in the LOS of the heroes?

By the RAW, you place all the monsters in the newly revealed area.